Crystalline spherules



United States Patent 26,294 CRYSTALLINE SPHERULES Harold G. Sowmau, Maplewood, and James R. Johnson,

White Bear Lake, Minn assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Deiaware No Drawing. Original No. 3,129,188, dated Apr. 14, 1964, Ser. No. 96,081, Mar. 16, 1961. Application for reissue June 23, 1966, Ser. No. 562,978

11 Claims. (Cl. 176-457) Matter enclosed in heavy brackets II] appears in the original patent but forms no part of this reissue specitication; matter printed in italics indicates the additions made by reissue.

This invention relates to spherules of refractory carbides and more particularly to spherules of thorium and uranium carbides and processes for the production thereof.

This application is a continuation-in-part of our copending application Serial No. 838,445, filed September 8, 1959, now abandoned.

A problem in the production of atomic reactors is that of providing reactor fuel elements containing uranium carbide as fissionable material, in which the said fissionable material is dispersed in small particles throughout a pellet or matrix. Desirably the particles are of substantially uniform size and shape in order to give the best possible results when incorporated into metallic matrices or employed in packed containers. While uranium carbide may of course be reduced by grinding or ball milling to particles of extremely small and fairly uniform size, although these may be of irregular shape, it is advantageous to provide particles which are predictably regular. Spherical particles are by far the most desirable in this connection. Furthermore, because of the pyrophoric nature of uranium carbide, minimization of the surface area by the provision of spherular particles is also very advantageous.

Fuel elements containing thorium carbide in combination with uranium carbide are useful for certain types of reactors. This combination appears to exist in the form of solid solutions in which the proportion of carbon is constant and the thorium-uranium ratio may vary but together they are stoichiometrically equivalent to the carbon. Compositions of this solid solution series are herein referred to as uranium thorium) carbide.

It is an object of this invention to provide small substantially spherical solid particles of crystalline, uranium carbide and uranium (thorium) carbide structures.

Still another object of the invention is to provide spherical particles which have surface coatings.

Other objects of the invention will become evident from the disclosures hereinafter made.

In accordance with the above and other objects of the invention, it has been found that solid spherules of boron carbide and uranium (thorium) carbide can be formed by rapidly melting discrete particles of these substances in admixture with a resilient inert isolating material of low bulk density such as amorphous carbon, graphite, boron nitride, and the like to form small molten spheres, and then cooling the mixture to solidify the carbides in spherical shape. The essential step appears to be that of maintaining the discrete, isolated particles of uranium (thorium) carbide in molten form for a time just sufficient to form spherules by operation of surface tension on the molten particles.

The uranium (thorium) carbide spherules according to the invention have the average composition U Th C where m and n are numbers from 0 to 1 and the sum of m and n is 1. For such compositions there appears to be a progressive series of solid solutions of increasing melting point from UC through U Th C to ThC Preferably, m is less than 1 and greater than zero.

By the term spherules as used herein, it is intended to designate substantially spherical structures having a diameter in the range of about 10 microns up to about mils, having a glossy surface formed from the molten or at least semi-molten state by the operation of surface tension, while the interior of the said spherules has a crystalline structure characteristic of the particular carbide system and is substantially free from voids. In some instances, the spherules when viewed under magnification appear to have a surface consisting of minute facets. The presence of such minute surfaces of different radius of curvature is immaterial when the particle as a whole is substantially spherical. The spherules exhibit the property of rolling on slight inclines which is characteristic of spheres.

Surprisingly, while the spherules produced in the process of the invention are crystalline as shown by X-ray diffraction studies, the surfaces of the spherules are uniformly smooth and substantially spherical, presenting a highly polished appearance when inspected under the microscope at moderate powers of magnification. The particles are furthermore solid, by which is meant that they are substantially free from voids. If particles of uniform size are used as starting materials, the resulting spherules are also of substantially uniform diameter. However, the starting material may contain a wide range of sizes, and the final product will in that case also have a wide variation in diameter. The spherules can of course be graded as to size by the use of appropriate sieves.

While it has heretofore been known to produce small beads from glass or vitreous materials, so far as is known the art has not heretofore been aware of any way in which spherules of hard crystalline substances, such as boron carbide, can be made. It is highly unexpected to find that crystalline spherules of uranium (thorium) carbide can in fact be produced without domination of the surface by crystal faces. Without being bound thereby, it may be hypothesized that the formation of spherules may result from a blanketing effect of the resilient or yielding finely divided carbon or other non-reactive low bulk density isolating media employed. When coarse isolating powders are used, of the order of particle size of the material which is to be made spherical, satisfactory spherules are not usually formed.

Broadly speaking, the process is carried out by isolating small irregularly shaped discrete particles of a source of uranium carbide, uranium (thorium) carbide or boron carbide by mixing them with an isolating medium of low bulk density, placing the mixture in a suitable furnace in the presence of a non-reactive atmosphere, rapidly heating the mixture to a temperature suflicient to form spherules owing to the surface tension forces acting on the molten or semi-molten carbide, cooling the mixture, and removing the isolating material from the spherules.

The isolating medium used can be any substance not inter-reactive with the carbides, which is of low bulk density, and which is not melted at, nor otherwise physically changed, at the temperatures used in the process. Examples of such materials are carbon and boron nitride in finely divided form.

Various compounds of uranium and boron, such as oxides, and the elements themselves may be employed as equivalents of the carbides, in that they form the respective carbides at the temperatures used, when carbon is employed as the isolating medium or in conjunction therewith. Furthermore, the corresponding compounds of thorium may be used with uranium compounds in any desired proportions, or in place thereof to furnish uranium thorium) carbide. Thus, sources of the uranium (thorium) carbide from which the spherules of the present invention are produced are uranium (thorium) carbides themselves, or, if carbon is employed, the metals, their oxides and the like. It is possible that the reaction with carbon which takes place when a source other than the final carbide is used as a starting material occurs more or less simultaneously with the spheroidization, and at a lower temperature than the melting point of the particular carbide. Without wishing to be bound by theory, it is possible that in some cases spherules of lower melting materials, such as oxide, are first formed, followed by rapid reaction with the carbon to form the carbide. However, it is considered that this process is fully equivalent to the process in which the carbide actually exists in molten form.

It will be apparent that if a precursor for the uranium (thorium) carbide, is used, such as the metals or their oxides, it will be necessary to provide at least sufficient carbon in the isolating mixture to react with the precursor in addition to the amount of isolant necessary to effect isolation of the particles.

To avoid oxidation, it is preferred to heat the mixture in a non-reactive atmosphere, meaning by the term atmospheres which do not react with any of the components of the mixture which is heated.

It is a rather surprising feature of the process that reaction to form spherules of carbides from sources other than uranium (thorium) carbides, e.g., the metals or oxides, proceeds so rapidly as to be virtually complete in the relatively short time intervals employed. This is particularly striking because as suggested above, reaction may be taking place with spherular particles of the precursor which then present a minimum surface for reaction.

While other finely divided isolating media may be employed, carbon powder is conveniently available and is used herein to exemplify isolating media in general as to the amounts and state of division which can be used. The carbon powder which is used need not be chemically pure carbon, but as used herein the term carbon is employed in the common technical sense and includes graphite, carbon black, lamp black, amorphous carbon, petroleum coke and the like low bulk density forms of carbon. Preferably, the particle size of the powdered carbon or other isolating particles which is employed is at least about one order of magnitude smaller than that of the material to be converted to spherules, so that there are a large number of carbon particles for each of the particles of carbide which is to be produced as a spherule. The effect is to provide a yielding, or resilient, supporting medium for the spherules as they form. Various forms of finely divided carbon and the particle sizes thereof are described in Industrial Carbon, Mantell, D. Van Nostrand Co., Inc., New York, 2nd ed., 1946.

Generally speaking, the proportion of isolant to the carbide or carbide precursor which is employed in the initial mixture may be varied over a wide range in carrying out the invention. Thus, 100 parts by weight of isolant can be used for each 1 to 100 parts of starting material, although it should be understood that proportions outside of this range can also be used. While an unduly high proportion of carbon powder results in somewhat less economical operation, in that the heating of a relatively large mass of material is required before fusion of the carbide particles takes place, this does not prevent formation of spherules. On the other hand, too small a proportion of carbon prevents the mechanical separating action to the degree which is necessary to obtaining good spheres and apparently also affects the resiliency adversely, It will be apparent that when a precursor substance is used which forms the selected carbide on heating with carbon the amount of carbon is increased to compensate for the carbon which is consumed in the reaction, or an amount of carbon must be added to the reaction mixture, as the carbon must serve as a carbon source for the chemical reaction to form the carbide. When an excess of carbon is present over the amount re quired to form the metal carbide, and the carbide particles are maintained at the melting point, carbon dissolves in the metal carbide. Some of this dissolved carbon remains in the sphernle on cooling, so that the spherule inherently contains carbon in excess of the amount of carbon combined in the form of metal carbide. Preferably, a ratio of about 5 to 20 parts of carbon by weight to 1 part by weight of carbide or carbide precursor is employed, as providing better size control for the spherules.

The spherules can be made by a batch process. In this case, after thorough mixing, to insure that the finely divided mixture of isolating medium and source of carbide is substantially homogeneous, it is placed in a suitable refractory container in a furnace, in an inert (non-reactive) atmosphere, and heated rapidly to a temperature in the range of about 2300 to 2700" C. As non-reactive atmospheres, argon, helium and the like may be mentioned as suitable, while nitrogen can be used but is somewhat less satisfactory owing to the possibility of the formation of nitrides. The chief function of the nonreactive atmosphere is to prevent the combustion of the carbon which would otherwise take place in the presence of oxygen at the temperatures employed, but the atmosphere used must not react with the carbide source or with the isolating medium. The temperature to be employed is that which is just sufiicient to melt the particular carbide which is selected, so that the force of surface tension can draw each particle of the carbide into a spherical shape. The melting point of boron carbide is about 2450 C. while that of uranium carbide is about 2350" C. and of thorium carbide about 2650 C. Although temperatures somewhat in excess of these temperatures can be employed, they are not necessary for successful formation of the spherules and may result in some coalescing of the spherules to form larger spherules. While this may not be undesirable vin some instances, it will be apparent that control of the diameter of the spherules will then be more difficult.

Temperatures of the order required in the process, i.e. above about 2000" C., are determined optically and therefore are not exact temperatures. Some variation in observed temperature from batch to batch may consequently be expected, as well as differences arising from personal visual errors.

The heating of the mixture is continued only for so long as is necessary to produce the spherules. The dwell time in the furnace, i.e., the length of time required to form spherules, is readily determined by empirical methods and depends on the mass of material being heated and the capacity of the furnace. The mass is kept in the furnace just slightly longer than required for the entire mass to reach the melting point of the carbide, which is just sufficient to bring about transformation of the carbide particles to spherules. The mixture is then rapidly cooled to a temperature below the melting point of the particular carbide involved, for example, by removal of the mixture, while maintaining the non-reactive atmosphere, to a zone in which positive cooling is accomplished. The rate of cooling is advantageously made high in the case of uranium (thorium) carbide in order to produce and maintain the fine crystalline structure of the dicarbide since under slow rates of cooling and in annealing there is some tendency for recrystallization with the formation of (U, Th) C and carbon. The effect is observed, for example, in etched metallographic specimens which show fine striae after annealing which are lacking in quenched spherules. Furthermore, there are differences in the unit cell dimensions as determined by X-ray diffraction techniques which indicate that the quenched material is more dense. After cooling, the spherules are separated from the remainder of the mixture, for example, by washing, screening, flotation techniques and the like.

Alternatively, a rotary kiln may be used for continuous larger volume commercial production bearing in mind the absolute necessity of avoiding critical masses. In this case, the mixture of isolant and starting materials is fed continuously into the upper end of a rotary kiln which slopes downwardly from inlet to outlet. The inlet end of the kiln is heated by any convenient means to a temperature in the appropriate range and as the mixture progresses it rapidly becomes heated to cause formation of spherules. A non-reactive atmosphere is provided in the kiln. The kiln may be provided with a positively cooled zone, or the mixture may be discharged into a stream of cold inert gas and allowed to fall into a receptacle. The slope of the kiln is adjusted to provide the proper dwell time for the temperature and heating unit employed, The use of a rotary kiln is less desirable since larger spherules are formed by agglomeration and size control is therefore more difficult.

In another method which can be used for making the spherules of the invention, a bed of the selected isolating medium is spread upon a refractory pallet, such as a graphite block of appropriate dimensions. The isolating medium can be present in a relatively thin layer, as for example about to 100 mils in thickness. The finely divided uranium or boron carbide is then sprinkled on this bed in a layer approximately one particle thick, and subjected to intense heat in a non-reactive atmosphere until the carbide particles are drawn into spherules. The pallet is thereupon removed from the heating zone and cooled.

It is commonly found that the isolating medium adheres to some extent to the surfaces of the spherules, particularly when carbon is employed. Preferably, the spherules are treated with a bath containing a detergent, to remove the carbon or other materials as completely as possible from their surfaces. However, where carbon is used, it will be apparent that for some applications it will not be necessary to remove the adherent carbon as completely as for others. If desired, substantially all of the carbon which may be adherent to the surface can be removed therefrom by oxidation under controlled conditions, as for example, by refluxing the particles with chromic acid solution, or by heating them in air at about 1000 C., in the case of boron carbide. For removal of particles of isolating material from uranium carbide spherules, if required, milling in a ball mill with an organic solvent such as acetone, and with rubber-covered steel balls, removes the adherent material.

Cleaning of the spherules of the invention is also usefully accomplished by employing ultrasonic vibrating devices. The process comprises suspending the spherules in an inert liquid having a viscosity no greater than that of water, preferably containing a small amount of surfactant, and subjecting the suspension to ultrasonic vibrations for about 3 to 5 minutes. The liquid is decanted and replaced with fresh liquid repeatedly until suspended carbon is no longer evident therein. Uranium (thorium) carbide spherules are cleaned by this procedure using nonhydroxylated solvents since reaction with hydroxylated solvents such as water may result in oxidation of the surfaces.

Having thus described the invention in broad general terms it is now more specifically illustrated by examples which show the best mode contemplated of practicing the invention. In these examples all parts are by weight unless otherwise specified.

EXAMPLE 1 A mixture of powdered boron carbide and graphite is prepared by mixing 6.75 parts of boron carbide of -l40 to +325 mesh and 33.75 parts of finely divided carbon (a furnace black or Thermatomic" carbon), in a one pint glass jar in which are placed coiled iron wires. Mixing is effected by rolling the jar on rollers of the type used for laboratory ball mills for 10 minutes at about 112 r.p.m. The mixture is separated from the wires avoiding manipulation which might cause separation of the solid ingredients and the mixture is packed loosely in a carbon tube of suitable size loosely fitted at both ends with threaded graphite plugs. This is referred to as a boat.

The boat containing the batch is placed at the entrance of a carbon tube furnace about 3 feet long and 3 inches in diameter approximately the central one-third portion of which is heated to a temperature of 2500 C. by passage of an electric current therethrough (resistance heating). Temperatures are determined optically. The furnace is flushed with argon to provide a substantially oxygen-free environment, i.e., a non-oxidizing atmosphere and to prevent oxidation of the carbon tube. Enough of the argon enters the boat to leave an inert atmosphere in the boat without special measures. After the boat has attained a bright red heat (about 700 C.) at the front of the furnace (about 2-3 minutes) it is moved into the central region of the furnace, held at 2500 C. and left there for about 8 minutes. When first moved to this region there is a drop in temperature of the furnace, which is quickly restored and the boat attains this temperature in about 3-5 minutes. The actual time required depends on the particular dimensions of the system. The slightly longer residence time of 8 minutes permits the boron carbide to melt and form molten spherules in the graphite matrix. The boat is then moved to the water cooled cold end of the furnace and permitted to cool rapidly to below a red heat (about 600 C.). This requires about 5 minutes. The boat is then removed from the furnace and one plug removed.

When cooled sulficiently that the batch inside no longer appears to glow, the batch is poured into a large volume of dilute solution of a detergent such as an alkyl ether of polyethylene glycol (available commercially under the trademark Tergitol from Union Carbide and Carbon Corporation). The suspension is stirred to permit wetting of the particles of carbon and spherules of boron carbide and poured through a 325 mesh wire screen. The residue is washed repeatedly with clear water and fresh detergent solution until the wash water no longer shows observable discoloration due to carbon. The batch is then dried and found to weigh slightly more than the boron carbide used due to a pickup of carbon partly as free occluded carbon and partly as carbon dissolved in the boron carbide. The spherules vary in diameter from about 50 to 200 microns, and are separated from a few percent of malformed particles by rolling down an inclined plane or other method. Screening serves to classify the spherules in various diameters if desired. Analysis for boron and carbon shows:

Percent Total B 67.74 Total C 34.23 Free C 12.09

The values for total B and C are presumably both slightly high. The atomic ratio of boron to combined carbon is about 3.4: to about 3.5 :1. The spherules show the characteristic X-ray diffraction patterns of boron carbide. These spherules are hard and smooth and are suited for incorporation in reactor designs as a burnable poison. Reference to the phase diagram shows that this composition (24.6 percent C combined) corresponds to the 13 phase.

EXAMPLE 2 The procedure of Example 1 is repeated except that 13.5 parts of carbon are used. Spherules of boron carbide having an approximate diameter ranging from 50 microns to 200 microns are obtained, while an amount of the boron carbide agglomerates to larger diameters. Similarly, when the procedure of Example 1 is repeated, except that 67.5 parts of carbon are used, spherules having the same diameter range are obtained. The use of 337.5

parts of carbon for isolating 6.75 parts or boron carbide likewise gives useful spherules, but the heating of this large mass of low bulk density requires an increase in dwell time in the furnace and the separation of the spherules from the carbon becomes somewhat more difficult. The change in the proportions of carbon employed does not substantially affect the amount of carbon which is found upon the surfaces of the spherules.

EXAMPLE 3 The procedure of Example 1 is repeated employing 1 part of uranium carbide about 70 to +150 mesh in place of the boron carbide, with about 3 parts by weight of thermatornic carbon. Because of the pyrophoric nature of uranium carbide it must be handled cautiously. The batch is heated for 10 minutes at 2350 C. after first permitting it to heat at 1000 C. for 4 minutes. The spherular uranium carbide is isolated from the batch after cooling as above by quickly washing the batch through a sieve which passes the carbon particles using water containing a small amount of detergent. Spherules of uranium carbide about l00-200 microns in diameter are obtained. While water can be used for separating the spherules from the isolating medium when small batches are processed, non-hydroxylatcd organic solvents, such as benzene, are preferred for use in processing large amounts of uranium carbide spherules to avoid the danger of decomposition. The spherules are dried in an argon atmosphere. The dry uranium carbide spherules are suitable for dispersion-type reactor fuel elements. The carbon content of the compound, in spherular form, is reduced from UC to predominantly UC by heating in dry hydrogen for 1 hour at 1300 C. This material is extremely pyrophoric and must be handled with care, under an inert atmosphere.

When powdered boron nitride is employed as an isolating medium for producing uranium carbide spherules, there is a possibility of contamination of the resulting spherules of uranium carbide, as with boron. Consequently, when uranium carbide spherules of high purity are required, the isolating medium of choice is finely divided carbon.

When 7.5 parts of uranium dioxide, about 100 to +200 mesh particle size, are carefully blended with 22.5 parts of carbon black as in Example 1, and then heated in an argon atmosphere to 1000 C. for 4 minutes, followed by minutes at 2350-2360 C., uranium carbide spherules are formed having diameters in the range of about 50 to 150 microns. These are separated from the isolating medium as before stated.

The preceding procedure is repeated, but using 7.5 parts of uranium dioxide of about Il00 to +200 mesh parti cle size, and 37.5 parts of finely divided carbon black and the batch, in an argon atmosphere, is kept at 1000 C. for 4 minutes and then maintained at about 2300-2375 C. for about 10 minutes. The batch is cooled and the spherules are separated as before. Uranium carbide spherules ranging from about 50 to 150 microns in diameter are obtained.

It is found that spherules are formed as above when the soaking, i.e., time of maintaining maximum temperature, is as short as ten minutes. It is somewhat advantageous to permit soaking to proceed for about to minutes in that there is better opportunity for spheroidization to proceed with better elimination of microscopic inclusions and voids. An annealing at a lower temperature, e.g., 2200 C., for two hours also serves to perfect the shape and density of the spherules.

When uranium dioxide is employed as the starting material it is advantageous to assure proper reaction by first mixing the uranium dioxide intimately with about 2.0 to 4.0 moles of carbon per mole of uranium dioxide and a small amount of an organic binder. A mixture of 100 parts of U0 (less than micron size), 9 parts of fine carbon and 4 parts of polyvinyl alcohol is ball milled for 2 hours to a homogeneous mixture. The dark gray mixture is dried and granulated to a size of about 250 to 350 microns and the granulated material is blended with an approximately equal weight of thermatomic carbon and spherulizcd as above by heating to 2525 C. for 30 minutes. The mixture is cooled, separated from the bulk of the supporting carbon by ultrasonic washing with benzene or isooctane with frequent decantation of the supernatant liquid and addition of fresh solvent. The cleaned particles no longer make a smudge when rubbed on uncalendered paper and are approximately to 250 microns in diameter. Metallographic examination of the spherules in section shows good crystalline structure and substantially circular outlines.

For certain nuclear reactor uses it is desirable that the spherules of uranium carbide or uranium (thorium) carbide, or the oxides, be sheathed with a tough cladding which is substantially impervious to fission products and yet non-absorptive of neutrons. Such coatings can be provided by plating the spherules with metals, by coating with appropriate ceramic powders together with a binder followed by firing, by decomposition of gases to deposit hard carbon on the spherules, or the like.

It is found that a particularly useful coating is provided by a layer of carbon deposited by pyrolysis of a hydrocarbon under certain conditions. Since the layer of pyrolytic carbon thus produced shows properties different from those of graphite or ordinary soft carbon it is convenient to refer to it by a distinctive term and it is herein termed pyrocarbon.

It is known that pyrocarbon coatings can be applied to relatively large articles, such as ceramic tubes and the like. However, the problem of applying a pyrocarbon coating to the spherules of the invention is rather more difiicult than the coating of tubular ceramic bodies since sequential mechanical movement and exposure as used heretofore is entirely unfeasible with such small objects. It has furthermore been found that somewhat higher temperatures are operative. This may be associated with a failure of the smooth substantially spherical surfaces of the spherules of the invention to provide any catalytic effect on the decomposition of the methane employed as a source of carbon, as compared with articles such as ceramic tubes which presumably have a rough and grainy surface.

It is found that pyrocarbon is deposited on the spherules as a tough coating by the pyrolysis of methane, carbon monoxide or the like at a temperature of about 1300 to 1700 C. Preferably a temperature in the range of about 1300 C. to 1450 C. is used. While some deposition of pyrocarbon occurs on static spherules, the preferred procedure is to maintain the spherules in motion to achieve a more uniform coating on each spherule. This is achieved by slowly dropping the spherules through a heated zone, by vibrating a suitable receptacle containing the spherules in a heated zone, by tumbling the spherules by rotation of the heated zone, or by fluidizing a bed of the spherules using an inert gas, for example, helium, at the temperatures noted above.

EXAMPLE 4 About 50 grams of spherules of UC (about 200 to 250 microns in diameter) prepared as described in the first part of Example 3 above are placed in a rotating graphite drum about 3 inches in diameter and 4 inches long having gas inlet and outlet connections rotatably mounted in a 3 /2 inch diameter quartz tube. A coil surrounding the tube is coupled with a 15 kw. 10 kc. generator so that the crucible is heated by induction. A stream of argon containing 10 percent by volume of methane is passed through the tube to displace air and the crucible and contents are heated to about 1300 to 1350 C. (determined by an optical pyrometer). Heating and rotation of the crucible are continued for about 1 hour and the How of methane is cut off and the crucible allowed to cool in the stream of argon. It is removed and the spherules are found to be coated with a tough hard layer of pyrocarbon about 30 microns thick.

EXAMPLE A mixture of 1 part of granules of pressed uranium oxide and carbon in 9:1 ratio and about 300 micron diameter and 2 parts by weight of Thermatomic carbon (finely divided, furnace black) is made by placing the ingredients in a twin shell blender and mixing thoroughly. A batch of desired amounts is packed loosely in a carbon tube (boat) which is loosely fitted at both ends with threaded graphite plugs and which is of suitable size to fit into the furnace used. The tube with the batch is permitted to heat at 1000 C. for four minutes. It is then moved to the central zone and there heated (fired) at 2550 C. Heating at this temperature for about 30 minutes produces substantially void-free spherules. After firing the batch-containing tube is moved to the end of the furnace, which is cooled with water and permitted to cool rapidly to below red heat. About five minutes are required for this cooling, whereupon the tube is removed from the furnace, one plug removed and the batch is poured into an argon allutriation separator in which the finely divided carbon is blowvn away from the larger spherules of uranium carbide with argon, in a continuous winnowing operation, and the spherules are very rapidly cooled. In this operation, nitrogen can also be employed as the inert gas, a conical vessel fitted for introduction of gas at the small lower end is provided with a foraminous (gas permeable) support near the same end on which the charge is placed. Passage of gas carries the fine particles away while the larger particles remain behind. The spherules which are obtained are about 100-200 microns in diameter. They are preferably stored in an inert atmosphere such as dry argon. Spherules thus obtained can be metal plated to provide a useful coating. Thus, for example, a quantity of UC spherules are placed in a copper dish which is made the cathode in an electroplating cell. A sheet of copper is used as the anode. The cell is filled with an electrolyte having the following composition:

H SO (cone) ml 37.0 CUSO4 The anode is located a short distance above the cathode. The spherules in the cathode are continuously agitated with a stream of argon flowing at a rate of about 23 cu. ft./hour. A current of about 0.45 ampere at 1.5 volts, is passed for ten minutes. At the end of that time the spherules are found to have a copper coating. This can be increased in thickness by continuing the plating process.

EXAMPLE 6 This example illustrates the formation of uranium (thorium) carbide spherules of the invention.

A mixture of 50 parts of -325 mesh thorium oxide, 20 parts of uranium oxide similar to that employed in Example 3, and 7 parts of furnace black (Thermax") are milled in a ball mill having alumina balls with 230 parts of water containing 0.5 part of Tergitol TMN" alkyl ether of polyethylene glycol, a surfactant, and 2.0 parts of polyvinyl alcohol as a binder to produce a creamy magma which is cast into a dish and dried at 75 C. in an oven for 24 hours to give a friable cake. The cake is ground to a powder in a mortar and pestle and screened through a 100 mesh screen to give a fine powdery material. This procedure of mixing is employed to obtain as homogeneous a material as possible since it is desired that the spherules made should each have the same composition. The powdery material is formed into pellets by pressing at 23,000 p.s.i. and is again pulverized in a mortar and pestle and screened and classified into particle sizes of 150 to 300 microns.

One part of the 150 to 300 micron size granules is combined and blended with 2 parts of carbon (thermatomic) for 30 minutes in a V blender. The entire mixture is placed in a graphite boat in a carbon tube furnace as described above and fired in a flowing argon atmosphere. Heating up to 2550 C. requiring A hour and a temperature of about 2550 to 2580" C. is then maintained for 30 minutes. The boat is removed to the cool end of the furnace and allowed to cool maintaining the stream of argon for one hour. The carbon including the spherules of uranium (thorium) carbide is suspended in dry isooctane and agitated in an ultrasonic cleaner for about three minutes. The spherules settle immediately, the agitation stops and the thick carbon suspension is decanted. This process is repeated about 3 times until the supernatant liquid is clear. The resultant uranium (thorium) carbide spherules, in which the ratio of thorium to uranium is about 5 to 2 (approximate formula U Th C are further classified to remove a few irregular particles and screened. They are substantially spherical particles of about to 200 micron diameter.

The spherules are coated with pyrocarbon in the apparatus and by the procedure described in Example 4 for uranium carbide spherules. The spherules are thus heated in argon containing 90 percent by volume of methane for 2 hours. The resultant spherules have a 60 micron thick coating of pyrocarbon and inherently have increased stability at elevated temperatures below the melting point of the metal carbide as a result thereof. They are suitable for use in graphite matrix fuel elements.

By repeating the above procedure employing various proportions of thorium oxide, spherules are formed having any desired ratio of thorium to uranium. When uranium is omitted the spherules formed are of thorium carbide. Thus, for example, by repeating the above procedure using 1 part of thorium dioxide and nine parts of uranium dioxide, the approximate formula of the resulting material is U Th C and when 9 parts of thorium dioxide and 2 parts of uranium dioxide are used, the approximate formula is U Th C As the proportion of thorium increases the temperature of firing is increased so that in the absence of uranium a temperature at least above about 2655 C., the reported melting point of ThC and ranging up to about 2900 C., is employed. Thus, by repeating the procedure set forth hereinabove, but using 70 parts of 325 mesh thorium oxide, there is produced spherular thorium carbide of 150-300 microns diameter. This may also be coated with pyrocarbon if desired.

What is claimed is:

1. As articles of manufacture, spherules having a diameter in the range of about 10 microns to about mils of crystalline material of the class consisting of uranium carbide, uranium (thorium) carbide and thorium carbide having a smooth, substantially spherical outer surface formed from the molten state by surface tension forces, and characterized by substantially void-free crystalline internal structure.

2. Spherules of crystalline uranium carbide according to claim 1.

3. Spherules of crystalline thorium carbide according to claim 1.

4. Spherules of crystalline uranium (thorium) carbide of the formula ber which is less than 1 and greater than zero, and the sum of n and m is 1; having a diameter in the range of about 10 microns to about 125 mils, having a smooth, substantially spherical outer surface formed from the molten state by surface tension forces, and characterized by substantially void-free crystalline internal structure.

5. As articles of manufacture, spherules having a diameter in the range of about 10 microns to about 125 mils of crystalline material of the class consisting of uranium carbide, uranium (thorium) carbide and thorium carbide having a smooth, substantially spherical outer surface formed from the molten state by surface tension forces, characterized by substantially void-free crystalline internal structure and having a continuous surface coating of pyrocarbon.

6. Pyrocarbon coated spherules of crystalline uranium carbide according to claim 5.

7. Spherules of crystalline uranium (thorium) carbide having a continuous surface coating of pyrocarbon, according to claim 5.

8. An improved metal carbide particle having a diameter in the range of about 10 microns to 125 mils, said particle comprising a body of metal carbide of the highest carbide form from the class consisting of uranium carbide, thorium carbide and uranium (thorium) carbide, said body having a smooth, substantially spherical outer surface formed from the molten state by surface tension 25 forces, and characterized by substantially void-free crystalline internal structure, and containing carbon in excess of the amount of carbon combined in the form of metal carbide: and an outer protective coating of carbon enclosing said body.

9. The improved metal carbide particle of claim 8 wherein said metal carbide comprises nuclear fuel carbide.

10. The improved metal carbide particle of claim 8 wherein said outer protective coating comprises pyrolytic carbon.

11. The improved metal carbide particle of claim 10 wherein an intermediate layer of graphite is disposed between said body and said outer coating.

References Cited The following references, cited by the Examiner, are of record in the patented file of this patent or the original patent. UNITED STATES PATENTS 3,010,889 11/1961 Fortescue et al. 176-58 X 3,141,829 7/1964 Fortescue et al 176-59 X 1,025,469 5/1912 Hunter. 1,501,419 7/ 1924 Poclszus. 2,027,788 1/1936 Ridgway et al.

2,510,574 6/ 1950 Greenhalgh. 2,580,349 12/1951 Fisher. 2,728,107 12/1955 Hershey. 15 3,041,260 6/1962 Goeddel.

FOREIGN PATENTS 756,014 8/1956 Great Britain. 792,114 3/1958 Great Britain. 1,037,605 8/ 1958 Germany. 1,206,300 8/1959 France.

OTHER REFERENCES Ridgway (11): Electrochemical Society Prep-rint 66-27 for release Oct. 1, 1934, pp. 295, 296, 300, 306, 307, 308. Accary: Nuclear Power, vol. 5, No. 50, pp. 122, 123, June 1960.

Nucleonics, vol. 14, March 1956, p. 36. Reactor Core Materials, vol. 4, No, 4, pp. 6, 7, 18, 19, November 1961. Finniston et al.:

Pergamon Press.

AEC Documents: BMI 1309, p. 2, Ian. 2, 1959', BMI 1357, pp. 12, 52, 53, 67-70, 80, July 1, 1959; BMI 1441, pp. 67, 69, May 31, 1960; BMI 1442, pp. P-l through P3,

July 12, 1960; ORNL 1633, pp. 1-10, Dec. 3, 1953.

Progress on Nuclear Energy (1956),

L. DEWAYNE RUTLEDGE, Primary Examiner. CARL D. QUARFORTH, Examiner.

Dedication Reissue N0. 26,294.-Har0Zd G. Sowman, Meplewood, and J amrs 1?. Johnson, White Bear Lake, Minn. CRYSTALLINE SPHICRULES. Patent dated Nov. 7, 1967. Dedication filed May 8, 1973, by the ussignee, the United States 0 f America.

Hereby dedicates t0 the Public of the United States the portion of the term of the patent subsequent to Feb. 12, 1973.

[Ofiicial Gazette N o'vember 27 1973.] 

